Previously, urethanes were generally prepared by the reaction of an aromatic isocyanate with an alcohol. The aromatic isocyanate was usually obtained by the reaction of phosgene with the appropriate primary amine. This primary amine, in turn, was generally obtained by reduction of the corresponding nitro compound. This conventional process has various disadvantages, not the least of which is the toxicity and corrosive nature of phosgene and the formation of hydrogen chloride as a by-product. Also, it is known that certain aromatic amines have harmful biological properties and some of them tend to be oxidized by air during storage.
There have, therefore, been several attempts to avoid the use of highly toxic phosgene and obtain urethanes directly from the corresponding nitro compounds, the corresponding alcohols and carbon monoxide. The processes according to U.S. Pat. No. 3,993,685 and British Pat. No. 1,472,243 use catalyst systems based on metals of the platinum group. Since the loss of very expensive catalysts is inevitable, these processes have not been used on a large commercial scale.
In the process according to U.S. Pat. No. 3,895,054, it is proposed to use a combination of selenium or sulphur or compounds of these elements with very large quantities of a base as the catalytically active system. The bases which may be used include, for example, triethylamine and pyridine. In order to obtain a satisfactory start to the reaction in the presence of these amines, it appears necessary to use rather large quantities of tertiary amines in relation to the nitro compounds used as the starting material. In fact, if dinitrotoluene is used as the nitro compound, the quantity of tertiary amine used is equal to, or greater than, that of the dinitrotroluene. The use of such large quantities of a tertiary amine entails numerous economic and recovery problems. Moreover, this process leads to the formation of by-products, such as amino compounds and ureas, if measurable quantities of water are present, e.g. as hydrates or in the free form. This process is, therefore, also unsuitable for large scale industrial application.
The reduction in the yield of the desired urethanes due to the above-mentioned formation of by-products can be prevented. According to British Pat. No. 1,485,108 the use of a catalyst system which is composed of elementary selenium or a selenium compound and a promoter consisting, for example, of a bicyclic amidine and a carboxylic acid, increases the yield. Although this process provides higher yields of urethanes it also leads to disturbing quantities of by-products which constitute the products of hydrolysis and secondary reactions of the urethane formed.
The process of U.S. Pat. No. 4,080,365 must be regarded as a further development in that the formation of by-products from urethane is suppressed through the use of aromatic urea compounds of aromatic amino compounds correspondings to these by-products. Although this measure provides an improvement it still has serious disadvantages. In particular, it requires the use of exceptionally large quantities of selenium or selenium compounds so that large quantities of this catalyst are lost. Moreover, selenium or the selenium compounds used as catalysts are not entirely acceptable toxicologically and in addition impart an unpleasant odor to the urethane produced.
It is therefore an object of the present invention to provide an improved process for the preparation of urethanes from aromatic nitro compounds, alcohols and carbon monoxide which could be carried out either entirely without selenium or selenium compounds or with much smaller quantities of these substances and still provide quantitive formation of urethane.